This invention pertains to bearing assemblies, in which to journal shafts, and in particular to lubrication means for the relatively movable components of bearing assemblies.
Long-life, hard-material journals and thrust collars, in bearing assemblies, which run against sleeves or shafts of same or similarly hard material, such as are required for handling abrasives, and high bearing energy loads in highly corrosive environments, i.e., journals, collars and sleeves of solid alpha sintered carbide, have a very poor dry run capability. Such bearing assemblies, used commonly in chemical, magnetic drive pumps, can experience premature, and quick bearing failures, during upset periods which result in lubrication starvation of the assemblies. Further, most magnetic drive pumps exacerbate a bearing temperature rise problem, during periods of dry running, because of additional heat from eddy currents generated in the typical metallic containment shell in which the bearings thereof are enclosed. When run dry, the bearings fail so quickly that any monitoring equipment or instrumentation therefor can not respond rapidly enough to halt the pump before serious damage is sustained by the bearings. These hard bearings, of solid alpha sintered silicon carbide, in addition, require very small running clearances, between the journal and the sleeve, i.e., approximately 0.0015 inch, per inch of sleeve diameter, to perform effectively. Consequently, movement of the sleeve within the journal is quite limited before surface contact loads are generated within the bearing. Any forces exerted against the journal or sleeve, by the lubrication means, must, therefore, not excessively overload the bearing.
Known prior art, as exemplified by U.S. Pat. No. 4,869,603, issued to Rudolf Meltzer, et al, on Sep. 26, 1989, for a Bearing for Cylinders of Printing Presses and Method of Making It, discloses the use of polytetrafluoroethylene as a lubricant for the bearing. Such is beneficial, in that polytetrafluoroethylene has a much higher thermal expansion rate than has the aforesaid bearing material, solid alpha sintered silicon carbide. When the bearing heats up, the aforesaid lubricant expands and wipes onto the bearing sleeve or thrust collar, coating the same, and resulting in the lubricant, polytetrafluoroethylene, running against itself.
As a lubricant, polytetrafluoroethylene, known commonly by its trade mark Teflon, offers the aforesaid benefit of coating the mating component and running upon itself, but in a bearing assembly requiring small running clearances, the Teflon expansion adds loading to the bearing assembly. Even the employment of individual lengths of Teflon, spaced apart within the relatively movable bearing components, as set forth in the aforesaid U.S. Pat. No. 4,869,603, would unduly load a bearing assembly which has small running clearances. Thermal expansion of the strips would have no means for any one thereof to yield due to expansion of others thereof. Too, in that the subject bearing will manifest diverse temperatures, circumferentially, the lengths or strips of Teflon can not adjust therefor, as to permit one strip to expand less than another, to insure against unduly loading the bearing. Further, the Teflon which is wiped onto the smooth, hard polished bearing surfaces for lubrication, will wear off with time and must, therefore, be reapplied during each period of heating from dry bearing operation. This would allow the bearing surfaces to wear until the Teflon is reapplied.
What has been needed is a lubricant means for a bearing assembly which is not met with the aforesaid difficulties, and is efficiently useful in bearing assemblies having small running clearances, and which assemblies are fabricated from hard materials.